Hydrostatic travelling mechanism

ABSTRACT

The invention relates to a hydrostatic traveling mechanism ( 4 ) that comprises a closed hydraulic circuit ( 6 ) in which a first hydraulic pump ( 4 ) with a first hydraulic engine ( 12 ) is linked with a flow pipe ( 7 ) and a return pipe ( 8 ). Said hydraulic engine ( 12 ) drives the vehicle wheels ( 14, 15, 18, 19 ) of two first vehicle axles ( 17, 20 ). According to the invention, an additional open hydraulic circuit ( 9 ) is used in which a second hydraulic engine ( 22 ) and a third hydraulic engine ( 24 ) are linked with a second hydraulic pump ( 5 ) via a supply pipe ( 11 ) and a control valve ( 21 ). Said second hydraulic engine ( 22 ) and said third hydraulic engine ( 24 ) drive the vehicle wheels ( 23, 25 ) of a second vehicle axle ( 36 ).

The invention relates to a hydrostatic drive for driving a vehicle, inparticular a commercial vehicle for use in forestry.

A hydrostatic drive according to the precharacterising clause of claim 1is known, for example, from DE 195 24 189 C2. In this known hydrostaticdrive, a hydraulic pump arranged with a hydraulic motor in a closedhydraulic circuit is driven by means of an internal combustion engine.Both the hydraulic pump and the hydraulic motor can be adjusted by anelectrically controlled adjusting device in dependence on electricalcontrol signals generated by an electrical control unit. The hydraulicmotor drives the vehicle wheels of a vehicle axle via a change-speedgear and a differential gear. The disadvantage of this hydrostatic driveis that the drive takes place exclusively via a closed hydrauliccircuit. The connection of the hydraulic motor to the hydraulic pump isrelatively rigid, so that in the event of load alternation, inparticular on uneven and rough terrain, spinning of the driving wheelsfrequently results. The mechanical differential gear has thedisadvantage that, on rough terrain, a differential lock is necessary.When the differential gear is locked, however, slipping of the drivingwheels when cornering inevitably occurs, resulting, particularly whenused in forestry, in damage to the ground, which is to be avoided as faras possible.

Mobile implements used on rough terrain, in particular forestryimplements, are frequently equipped with two or more driven vehicleaxles, a vehicle joint allowing an angular offset between the drivenvehicle axles in order to give the mobile implement the necessaryoff-road mobility. In this case, the driving of the vehicle axles is aparticular problem. A known solution is to connect the vehicle axles bya cardan shaft. However, this involves a relatively high degree ofstructural complexity. The rigid coupling of the vehicle axles has thedisadvantage of increasing soil erosion. Moreover, the maximum angularoffset of the driven vehicle axles is limited by the cardan shaft.

Furthermore, it is known to drive the driven vehicle axles by means ofseparate hydraulic motors which are connected to a common hydraulic pumpby a closed hydraulic circuit. Here, too, there is the disadvantage ofundesirably high soil erosion due to the rigid hydraulic coupling. Inmobile implements where the main function consists in the hydraulicoperation of tools and where the hydrostatic drive is merely assigned anauxiliary function, it is known to operate the hydrostatic driveexclusively in an open hydraulic circuit. The disadvantage of this,however, is the low efficiency and the low torque which can begenerated.

The object on which the invention is based therefore is to specify ahydrostatic drive in which both high efficiency and high torque areachieved with little damage to the ground.

The object is achieved by the characterising features of claim 1 inconjunction with the generic features.

According to the invention, a hydrostatic drive is provided whichcombines the advantages of a drive via an open hydraulic circuit withthe advantages of a drive via a closed hydraulic circuit. According tothe invention, two drive trains are therefore provided. In a first drivetrain a first hydraulic pump cooperates with a first hydraulic motor ina closed hydraulic circuit, whereas in a second drive train a secondhydraulic pump cooperates with a second hydraulic motor in an openhydraulic circuit. The two hydraulic motors in this case drive differentvehicle wheels, preferably different vehicle axles. A relatively hightorque can be achieved with the drive train having a closed hydrauliccircuit. In the drive train having the open hydraulic circuit thehydraulic motor is coupled relatively loosely to the hydraulic pump,enabling the rotational speed of the vehicle wheels driven by this drivetrain to adapt dynamically to the circumstances defined by the conditionof the ground and the obstacles on the ground. As a result, account istaken of the fact that the rotational speeds of the two differentlydriven vehicle axles deviate from one another considerably when, forexample, one of the two vehicle axles negotiates an obstacle. If the twovehicle axles are rigidly coupled to one another hydraulic ormechanically, as in the prior art, this would lead to slipping of one ofthe two vehicle axles, which inevitably causes damage to the ground.Through the relatively loose coupling, according to the invention, ofthe hydraulic motor to the hydraulic pump in one of the two drivetrains, the rotational-speed differences described are for a short timeequalised in the open hydraulic circuit. Since one of the vehicle axlesis driven via a closed hydraulic circuit, however, a high torque can begenerated nevertheless.

The subclaims relate to advantageous developments of the invention.

It is advantageous to arrange a control valve in the forward-flow lineof the open hydraulic circuit. Preferably, two hydraulic motors drivingopposite vehicle wheels of a vehicle axle are connected to the controlvalve of the open hydraulic circuit. The control valve then makes itpossible for these hydraulic motors to be apportioned individualvolumetric flows, so that given suitable control of the control valve adifferential action can be achieved. At the same time, the apportioningof different volumetric flows to these two hydraulic motors also allowsthe vehicle to be steered. If the vehicle axles are not rigidly alignedwith one another, as is customary in commercial vehicles for use inforestry, but rather the vehicle body is provided with a joint whichallows a kink angle between the vehicle axles, then this kink angle canbe increased or decreased in a specific manner by differential drivingof the hydraulic motors connected to the control valve.

Furthermore, in the drive train having an open hydraulic circuit, thehydraulic motors provided there can be switched off during high-speedoperation, for example on a forest road, so that the drive then takesplace only via the closed hydraulic circuit. This has the advantage thatthe hydraulic motors operated in the open circuit do not have to run ata high rotational speed as well.

The open hydraulic circuit can, furthermore, be used at the same timefor driving further hydraulic components, for example a saw, crane, etc.without an additional hydraulic pump being necessary for driving thesefurther hydraulic components.

The invention is described in more detail below with reference to thedrawing, in which:

FIG. 1 shows a basic hydraulic circuit diagram of a first exemplaryembodiment of the invention;

FIG. 2 shows a basic electronic circuit diagram of the first exemplaryembodiment of the invention;

FIG. 3A shows a first part of a hydraulic circuit diagram of a secondexemplary embodiment of the invention; and

FIG. 3B shows the second part of the hydraulic circuit diagram of thesecond exemplary embodiment of the invention.

FIG. 1 shows the components of the hydrostatic drive 1 required tounderstand the invention in a basic hydraulic circuit diagram.

A internal combustion engine 2, for example a diesel engine, is used todrive a first hydraulic pump 4 and a second hydraulic pump 5 via acommon drive shaft 3. The hydraulic pumps 4 and 5 can be designed, forexample, as a compact double pump. According to the invention, thehydrostatic drive 1 has both a closed hydraulic circuit 6 with aforward-flow line 7 and a return-flow line 8 and an open hydrauliccircuit 9. In the open hydraulic circuit 9, the hydraulic pump 5 drawsthe hydraulic fluid out of a hydraulic fluid tank 10 and feeds it into asupply line 11. Various hydraulic consumers are connected to the supplyline 11 and cause the hydraulic fluid to flow back into the hydraulicfluid tank 10.

In the closed hydraulic circuit 6, a first hydraulic motor 12 issituated between the forward-flow line 7 and the return-flow line 8 anddrives the vehicle wheels 14 and 15 of a first vehicle axle 17 and thevehicle wheels 18 and 19 of a third vehicle axle 20 via a mechanicaldrive train 13. In the closed hydraulic circuit, the function of theforward-flow and return-flow line is reversed when the direction ofrotation of the hydraulic motor 12 is reversed. In the exemplaryembodiment, the vehicle wheels 14 to 19 form the front wheels of thevehicle to be driven, for example a commercial vehicle for use inforestry.

In the open hydraulic circuit 9, a control valve 21 is situated betweenthe supply line 11 and the hydraulic fluid tank 10. A second hydraulicmotor 22, which drives a vehicle wheel 23, and a third hydraulic motor24, which drives a vehicle wheel 25, are connected to the control valve21 via hydraulic circuits 26 and 27. The hydraulic circuits 26 and 27are designed as open hydraulic circuits in the exemplary embodiment.

The control valve 21 is connected via an electrical control line 28 to adrive controller 29, which is connected to adjusting devices of thehydraulic pumps 4 and 5 and an adjusting device of the hydraulic motor12 via further electrical control lines 30, 31 and 32. The drivecontroller 29 is controlled by the operator via a control transmitter33.

The drive controller 29 can operate as a follow-up controller, the driveof the rear wheels 23 and 25 taking place in dependence on the drive ofthe front wheels 14, 15, 18 and 19. The lever 34, for example ajoystick, is used to preset the vehicle travelling speed desired by theoperator or the desired rotational speed n′ corresponding to thisvehicle speed. In the electronic drive controller 29, this desiredrotational speed n′ is compared with the actual rotational speed ndetermined by a sensor 35, for example, on the third vehicle axle 20. Byappropriately controlling the adjusting devices of the hydraulic pump 4and of the hydraulic motor 12 in the closed hydraulic circuit 6, theactual rotational speed n is adjusted to the desired rotational speedn′. Since the coupling of the hydraulic motor 12 to the hydraulic pump 4in the closed hydraulic circuit 6 is relatively rigid, a relatively hightorque can be transmitted to the vehicle wheels 14, 15, 18 and 20 inthis drive train.

The control valve 21 in the open hydraulic circuit 9 is controlled bythe drive controller 29 in such a way that the hydraulic motors 22 and24 are apportioned such a volumetric flow that the driving wheels 23 and25 rotate in a substantially slip-free manner. The volumetric flowspreset separately for the second hydraulic motor 22 and the thirdhydraulic motor 24 by the control valve 21 correspond to the rotationalspeeds n1″ and n2″ of the vehicle wheels 23 and 25, respectively. If anobstacle, for example a tree trunk, occurs on the route and thereforethe rotational speeds n on the one hand and n1″ and n2″ on the otherhand, determined by the drive controller, for a short time whennegotiating the obstacle do not match the calculated values, such ashort-time difference in the rotational speeds is equalised by thehydrostatic drive according to the invention, since the hydraulic motors22 and 24 of the open hydraulic circuit 9 are coupled relatively looselyto the hydraulic pump 5. A high degree of slip, which would lead todamage of the ground, does not occur.

When cornering, the two hydraulic motors 22 and 24 are apportioned, bysuitable control of the control valve 21 by the drive controller 29, avolumetric flow which differs in such a way that differentialequalisation on the second vehicle axle 36 is taken into account. Adifferential gear and a differential lock are not necessary. Adifferential lock which may be present on the first and third vehicleaxle 17 and 20 does not generally have to be actuated, since the vehiclewheels 23 and 25 of the second vehicle axle 36 are constantly lockedrelative to one another. This reduces damage to the ground.

The vehicle axles 17 and 20 of the front wheels 14, 15, 18 and 19 can bepivoted by a pivot angle α in relation to the vehicle axle 36 of therear wheels 23 and 25 in order to achieve a greater off-road mobility ofthe vehicle. To this end, an appropriate joint is provided on thevehicle body (not illustrated). The actual kink angle α formed by thevehicle axles 17 and 20 in relation to the vehicle axle 36 can bedetected by a sensor 37 and evaluated by the drive controller 29.Through the vehicle cornering control, which can be influenced by thelever 34 of the control transmitter 33, a desired kink angle α′ isspecified and is likewise supplied to the drive controller 29. Thecontrol valve 21 is then supplied with a control signal via the controlline 28 in such a way that during the apportioning of the volumetricflows for the two hydraulic motors 22 and 24 the kink angle α and theresultant radius of curve are taken into account. To match the actualkink angle α to the desired kink angle α′, an additionalrotational-speed difference can be generated in the hydraulic motors 22and 24 via the control valve 21, so that the control valve 21 enablessteering of the vehicle.

The open hydraulic circuit 9 can be used at the same time for drivingfurther hydraulic components, for example a crane, a saw or a harvesterhead. These further hydraulic components are schematically indicated inFIG. 1 by the reference numeral 38. Advantageously, a further hydraulicpump is not necessary for driving these further hydraulic components 38,the second hydraulic pump 5 being used both for the hydrostatic partialdrive by means of the open hydraulic circuit 9 and for driving theseadditional hydraulic components 38.

FIG. 2 shows a basic electronic circuit diagram of the hydrostatic drive1 according to the invention corresponding to the exemplary embodimentillustrated in FIG. 1.

In the exemplary embodiment, the control transmitter 33 comprises twocontrol levers 34 a and 34 b which are connected via an electrical bussystem on the one hand to the drive controller 29 and on the other handdirectly to various hydraulic components 50 a to 50 g of the vehicle tobe driven, which perform working functions on the crane, such asslewing, lifting and also actuation of jib, telescope, crane base andsteering. These hydraulic components 50 a to 50 g are, for their part,connected to the supply line 11 of the open hydraulic circuit 9preferably via load-sensing valves. Control via the bus system isrelatively flexible and easy to construct by comparison with a hydrauliccontrol.

The drive controller 29 consists of the actual control component 29 aand the vehicle electrics 29 b. The vehicle electrics 29 b haveinputs/outputs 53 and 54, to which, for example, the sensors 35 and 37can be connected. However, it is also possible, for example, to connectlighting components of the vehicle or other operating components tothese inputs/outputs 53, 54. The vehicle electrics 29 b have a display51, by means of which the functions of the vehicle can be monitored bythe operator. All the components are interconnected via bus systems. Thedrive controller 29 is used, as described, to control the drive of thefront axles 17, 20 and of the rear axle 36 via the hydraulic pump 4 andthe hydraulic motor 12 of the closed hydraulic circuit 6, and via thecontrol valve 21 of the open hydraulic circuit 9, respectively.Furthermore, the pivot angle of the second hydraulic pump 5 serving asworking pump is specified by a corresponding electronic control signal.The harvester head 52 is also controlled in this way. To this end,electronically controllable control receivers 57-59 are provided. Thecontrol receiver 57 is used to control the drive, the control receiver58 is used to control the steering and the control receiver 59 is usedto control the second hydraulic pump 5.

The electronic control components ensure a high degree of flexibility.As a result of the modular construction, the system can be readilyexpanded for new components.

FIGS. 3A and 3B show an exemplary embodiment of the drive according tothe invention in a more detailed representation than FIG. 1. Tofacilitate association, components which have already been described areprovided with corresponding reference numerals.

An internal combustion engine, not illustrated in FIGS. 3A and 3B,drives the drive shaft 3 and thus the first hydraulic pump 4 and thesecond hydraulic pump 5. The second hydraulic pump 5 feeds the hydraulicfluid, drawn from the hydraulic fluid tank 10, into the supply line 11.The control valve 21, which supplies the volumetric flow individually tothe second hydraulic motor 22 and the third hydraulic motor 24 via theauxiliary circuits 26 and 27, is situated in the supply line 11 of theopen hydraulic circuit 9.

The closed hydraulic circuit 6 comprises the hydraulic pump 4 and thehydraulic motor 12, which are connected via the lines 7 and 8. Anauxiliary pump 60, which feeds hydraulic fluid into the line 7 or 8,when conducting low pressure, via a pressure control valve 61 and afilter 62 and also one of the two nonreturn valves 63 and 64, issituated on the drive shaft 3. For pressure control of the line 7 and 8,when conducting high pressure, two pressure control valves 65 and 66respectively are used. The adjusting device 67 of the hydraulic pump 4is electromagnetically adjusted via a pilot valve 68 by means of twoelectromagnets 69 and 70. In a corresponding fashion, the adjustingdevice 71 of the first hydraulic motor 12 is electrically controlled viaan electromagnetically adjustable control valve 72. A two-stagechange-speed gear 73 is situated between the first hydraulic pump 12 andthe vehicle wheels 14, 15, 18 and 19 in order to increase the efficiencyof the drive train via the closed hydraulic circuit 6. Connecteddownstream of the mechanical change-speed gear 73 is a differential gear74. Part of the drive controller 29 is of hydraulic design in theexemplary embodiment illustrated, two control outputs 75 and 76 of thedrive controller 29 controlling the change-speed gear 73. One controloutput 77 of the drive controller 29 controls a braking device 78 of thefront vehicle axles 17 and 20, whereas two control outputs 79 and 80control two braking devices 81 and 82 of the second hydraulic motor 22and of the third hydraulic motor 24, respectively. The braking devices78, 81 and 82 are used to stop the drive rapidly during vehicle braking.

The invention combines the advantages of a drive having a closedhydraulic circuit 6 with the advantages of a drive having an openhydraulic circuit 9. Besides the cornering control, the drive controller29 can also take over the function of the level control of the drivingwheels. The hydraulic pumps 4 and 5 are preferably dimensioned in such away that they provide a sufficient delivery even at low rotationalspeed, in order to avoid unnecessary noise generation. The functions ofthe hydrostatic drive and the functions of the additional hydrauliccomponents 38 and 50 a to 50 g can be provided with preset priorities,so that when the maximum delivery rate of the hydraulic pump 5 isreached only certain preferred functions, for example the drive and thecornering control, are maintained.

What is claimed is:
 1. Hydrostatic drive (1) with a first hydraulic pump(4) and with a first hydraulic motor (12) which is connected to thefirst hydraulic pump (4) in a closed hydraulic circuit (6) via aforward-flow line (7) and a return-flow line (8) an drives at least onefirst vehicle wheel (14, 15, 18, 19), characterized by a secondhydraulic pump (5) and a second hydraulic motor (22) which is connectedto the second hydraulic pump (5) in an open hydraulic circuit (9) via asupply line (11) and drives at least one second vehicle wheel (23), thereturn of the hydraulic fluid from the second hydraulic motor (22) tothe second hydraulic pump (5) taking place via a hydraulic fluid tank(10), a control valve (21) which supplies the second hydraulic motor(22) with a variable volumetric flow dependent on a control signalcontrolling the control valve (21) being arranged in the supply line(11) of the open hydraulic circuit (9), said first hydraulic motor (12)driving the vehicle wheels (14, 15) of at least one first vehicle axle(17), wherein besides the second hydraulic motor (22) a third hydraulicmotor (25) is connected to the control valve (21), the second hydraulicmotor (22) and the third hydraulic motor (24) driving opposite vehiclewheels (23, 25) of a second vehicle axle (36), and the control valve(21) supplying the second hydraulic motor (22) and the third hydraulicmotor (23) with separate, variable volumetric flows in dependence on thecontrol signal.
 2. Hydrostatic drive according to claim 1, wherein thesecond and third hydraulic motors (22, 24) are connected to thehydraulic fluid tank (10) via the control valve (21).
 3. Hydrostaticdrive according to claim 1, the control valve (21) having a drivecontroller (29) connected thereto which, in dependence on a detectedactual kink angle (a) between the first vehicle axle (17) and the secondvehicle axle (36) and/or a detected actual rotational speed (n) of atleast one of the vehicle axles (17, 36), controls the control valve (21)such that the latter supplies the second and third hydraulic motor (22,24) with such separate volumetric flows that the vehicle wheels (23, 25)of the second vehicle axle (36) rotate in a slip-free manner. 4.Hydrostatic drive according to claim 3, wherein in dependence on adesired kink angle (α′), which is presettable through a controltransmitter (33), between the first vehicle axle (17) and the secondvehicle axle (36) and/or a desired rotational speed (n′) which ispresettable, the drive controller (29) controls the control valve (21)such that the latter supplies the second and third hydraulic motor (22,24) with such separate volumetric flows that the actual kink angle (α)approaches the desired kink angle (α′) and/or the actual rotationalspeed (n) approaches the desired rotational speed (n′).
 5. Hydrostaticdrive (1) with a first hydraulic pump (4) and with a first hydraulicmotor (12) which is connected to the first hydraulic pump (4) in aclosed hydraulic circuit (6) via a forward-flow line (7) and areturn-flow line (8) an drives at least one first vehicle wheel (14, 15,18, 19), characterized by a second hydraulic pump (5) and a secondhydraulic motor (22) which is connected to the second hydraulic pump (5)in an open hydraulic circuit (9) via a supply line (11) and drives atleast one second vehicle wheel (23), the return of the hydraulic fluidfrom the second hydraulic motor (22) to the second hydraulic pump (5)taking place via a hydraulic fluid tank (10), a control valve (21) whichsupplies the second hydraulic motor (22) with a variable volumetric flowdependent on a control signal controlling the control valve (21) beingarranged in the supply line (11) of the open hydraulic circuit (9),wherein above a limit speed of a vehicle driven by the hydrostatic drive(1), the control valve (21) switches hydraulic motors (22, 24) connectedthereto into freewheel operation and the drive then takes placeexclusively via the closed hydraulic circuit (6).
 6. Hydrostatic drive(1) with a first hydraulic pump (4) and with a first hydraulic motor(12) which is connected to the first hydraulic pump (4) in a closedhydraulic circuit (6) via a forward-flow line (7) and a return-flow line(8) an drives at least one first vehicle wheel (14, 15, 18, 19),characterized by a second hydraulic pump (5) and a second hydraulicmotor (22) which is connected to the second hydraulic pump (5) in anopen hydraulic circuit (9) via a supply line (11) and drives at leastone second vehicle wheel (23), the return of the hydraulic fluid fromthe second hydraulic motor (22) to the second hydraulic pump (5) takingplace via a hydraulic fluid tank (10), wherein further hydrauliccomponents (38, 50 a-50 g) are driven by the open hydraulic circuit (9).7. Hydrostatic drive according to claim 6, wherein the first hydraulicpump (4) and the second hydraulic pump (5) are driven by a commoninternal combustion engine (2).
 8. Hydrostatic drive according to claim7, wherein said further hydraulic components (38, 50 a-50 g) arehydraulic components of a crane.